1. Field of the Invention
The present invention relates to a radio frequency (RF) coupling circuit including both a transformer and a parallel AC-coupling capacitor.
2. Related Art
In a cascaded amplifier, AC-coupling must often be used between stages because the DC bias of the output of one stage output does not always match the desired DC bias of the input of the next stage. Often, a capacitor provides the AC-coupling and the DC bias of the next stage is provided by a resistor. Unfortunately, this technique may require a large AC-coupling capacitor if the next stage exhibits a low impedance. Moreover, undesired substantial attenuation of the signal may result. Additionally, a resistor for the bias element cannot be implemented in some techniques that require fast modulation of the DC bias of the next stage in a feedback loop (for example dynamic bias), because the resulting RC-delay can destabilize the feedback loop.
Alternatively, an asymmetric transformer for inter-stage coupling can be used. However, an asymmetric transformer operating at RF frequencies may be very difficult to design in both theory and physical implementation.
Therefore, a need arises for an RF coupling circuit that can prevent substantial attenuation of a signal, prevent de-stabilization of a feedback loop, and simplification of the circuit design process.